Demographics, clinical characteristics, and outcomes in hospitalized patients during six waves of COVID‑19 in Northern Iran: a large cohort study

Since the first report of coronavirus disease 2019 (COVID-19) in Iran, our country has experienced several waves of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. Northern Iran was one of the most affected regions of the country by COVID-19. In the current study, the demographic and clinical characteristics and outcomes of hospitalized patients were determined over a 2-year period (during six waves of SARS-CoV-2). This is a large cohort study investigating hospitalized patients with suspected and probable, and confirmed SARS-CoV-2 infection in Babol district, northern Iran, during the two years of COVID-19. The study population included patients admitted to four hospitals affiliated with Babol University of Medical Sciences between March 7, 2020 (start of the first wave) and March 20, 2022 (end of the sixth wave). Epidemiological and demographic characteristics, real-time PCR, cycle thresholds, clinical data and outcomes of COVID-19 were analyzed in 24,287 hospitalized patients. A total of 24,287 hospitalized patients were included in the study: 13,250 (46.6%) patients were suspected of having COVID-19, 11037(45.4%) were confirmed COVID-19 cases. The mean age of confirmed COVID-19 patients was 54.5 ± 18.9 years and 5961 (54%) were female. The median length of hospitalization for COVID-19 survivors and non-survivors was 5 (interquartile range [IQR] 4-8) and 7 (IQR 3-15) days, respectively. Of the patients with confirmed COVID-19, 714 (6.5%) died during hospitalization. In addition, the mortality rate from the first to the sixth wave was 22.9%, 8.1%, 9.9%, 6.8%, 2.7% and 3.5% in confirmed COVID-19 patients. The patients in the fifth wave were significantly younger than the others (mean age and SD of 51.1 ± 17.4 versus 59.2 ± 16.9, 54.7 ± 19.9, 58.4 ± 17.9, 53.5 ± 16.8 and 58.5 ± 25.1 years; p<0.001). The highest in-hospital mortality rate was 22.9% (126/551) in the first wave and the lowest in the fifth wave was 2.7% (96/3573) of cases. In conclusion, in the present study, the in-hospital mortality rate was 6.5% and more than half of the deceased patients were ≥65 years old. Male gender, advanced age and comorbidities significantly increased the mortality rate. The patients in the fifth wave were significantly younger than those in the other waves, and the lowest mortality rate and intensive care unit admission were also observed in the fifth wave.


Study design
The province of Mazandaran is one of the 31 provinces of Iran and is located in the north of the country near the Caspian Sea.According to the 2021 census, it is the 7th most populous province in Iran with 3,375,900 inhabitants.Mazandaran Province is one of the most populous and densely populated regions in Iran with 22 counties.Babol district is the most populous district in Mazandaran province with 546,800 inhabitants.Babol is the center of medical care in Mazandaran province and receives many patients from neighboring counties or provinces every year (Fig. 1).
All patients hospitalized between March 7, 2020 and March 20, 2022 suspected or laboratory-confirmed COVID-19 in four university hospitals in Babol district were included in this retrospective multicenter large cohort study.All hospitalized patients were followed up until discharge or death.The inclusion criteria were hospitalizations with suspected, probable or confirmed diagnosis of COVID-19 during the study.

Patient and public involvement
Patients and/or the public were not involved in the design, planning, management, and conduct of the research.

Ethics approval and consent to participate
The present study was conducted in accordance with the guidelines of the Declaration of Helsinki and all procedures involving human subjects were approved by the Ethics Committee of Babol University of Medical Sciences (No. IR.MUBABOL.HRI.REC.1401.271).All participating patients gave written informed consent before participating in the study.

Results
Over a period of two years, a total of 24,287 patients with suspected COVID-19 and a mean age of 52.4 ± 23.9 years (median: 56, IQR: 38-70) were enrolled in the study (46.7% female).Moreover, 11,037 laboratory-confirmed SAR-CoV-2 patients with a mean age of 54.5 ± 18.9 years (median, IQR: 56, 42-68) were included in the present study (54% female).Of all confirmed SARS-CoV-2 cases, 7197 (65.2%) were in the 18-64 age group.Tables 1 and  2 show the demographic and clinical characteristics of 24,287 and 11,037 suspected and confirmed COVID-19 patients included in this study, respectively.
The first wave took place between early March and mid-May 2020.The largest average number of daily admissions and in-hospital mortality was 37 (patients/day) in the fourth wave and the largest average number of daily in-hospital mortality was three (patients/day) in the first wave.The fifth wave started in late June and early July, followed by a gradual increase with a large number of patients admitted in early and late December.The average number of in-hospital mortality and daily admissions was 1 and 33 cases respectively in the fifth wave and 0.7 and 34 cases respectively in the sixth wave.March 20, 2022). 1 Null wave: related to between waves of SARS-CoV-2 infections (about 208 days), 2 CVD cardiovascular diseases, 3 KD kidney diseases, 4 BND brain & neurologic disorders, 5 RD respiratory diseases, 6 GID gastrointestinal diseases, 7 LD liver diseases, 8 HBD hematopoietic & blood disorders and 9 Others: diseases of immunodeficiency, lupus, special diseases, and thyroid.   2 and 4).In-hospital mortality was 1%, 15.1%, 27%, 32.2%, and 24.6% in the age groups <18 years, 18-49 years, 50-64 years, 65-79 years and ≥ 85 years, respectively.In addition, the ICU admission rate was 3.7% (891 patients) and  4 RD respiratory diseases, 5 GID gastrointestinal diseases, 6 LD liver diseases, 7 HBD hematopoietic & blood disorders, 8 Others: including diseases of immunodeficiency, lupus, special diseases, and thyroid, 9 Null Wave: related to between waves of SARS-CoV-2 Infections (about 208 days).3 and 4).Additionally, more women (54%; 5965/11045) were hospitalized with a positive rRT-PCR test, while more men died (men: 7.4%; 380/5080 vs. women: 5.6%; 334/5965) (P < 0.001).

Survival analysis
Cox regression was used to assess the factors influencing patient survival.In the first step, all variables including age, gender, rRT-PCR result, mean Ct value, underlying diseases and different waves were entered into the model.The coefficients were estimated using the stepwise backward method (likelihood ratio).In step eight, the variables age, gender, rRT-PCR result, mean Ct value, KD, hypertension, malignant disease, respiratory disease (RD) and liver as underlying disease as well as waves remained in the model.
Table 5 illustrates that the mortality risk for positive rRT-PCR cases is 0.38 higher than for negative cases (HR = 1.38, 95% CI 1.16-1.64).Moreover, the risk of mortality was higher in patients with mean Ct: 9-20 than in patients with mean Ct: 31-40 (HR = 1.93, 95% CI 1.56-2.39),which is almost twice as high as in patients with mean Ct: 31-40 and negative rRT-PCR.The risk of death therefore also increases with increasing viral load.Furthermore, the fifth and sixth waves had a lower mortality risk compared to the null waves and seemed to have protected against death (like hypertension, which protected against 30% of the risk).As can be seen in Fig. 2, patient survival (length of hospitalization to discharge alive or dying) was significantly higher in the fifth and sixth waves (Delta and Omicron) than in the null wave, and survival in the first wave (Wuhan) was significantly lower (Fig. 2).

Discussion
This study investigated the demographic, epidemiological and rRT-PCR findings and outcomes of hospitalized COVID-19 patients in Babol district of northern Iran during the two years of the pandemic.In the current study, 24,287 patients with suspected COVID-19 and 11,045 patients with confirmed SARS-CoV-2 infections were admitted to the hospitals.CVD, diabetes, BND, cancer, KD, RD, pregnancy, age, gender and pandemic waves with risk factor(s) were associated with COVID-19 mortality.
Therefore, a decreasing trend in the mortality rate of patients was observed over the course of the study, while the highest mortality rate was recorded in the first wave of SARS-CoV-2.In the current study, 4.4% of patients with rRT-PCR confirmed COVID-19 were admitted to the ICU and in-hospital mortality was 6.5%.In a national retrospective cohort study in Iran between February and April 2020, in-hospital mortality within 30 days was 24.4% (5693 of 23,367 patients) 20 .In a study in the province of Yazd, central Iran, the mortality rate was 9.8% (2185/ 24,563) 19 .In a nationwide study of Brazilian hospitalized patients with rRT-PCR confirmed COVID-19, 59% (4002 of 79,687) of patients were admitted to the ICU and in-hospital mortality was 38% (87,515 of 232,036 patients) 22 .The in-hospital mortality rate for COVID-19 patients in India 23 , Oman 24 , and the USA 25 was 13.7%, 21.4%, and 26%, respectively.Although hospital admission conditions, patient characteristics, case descriptions, etc. differ from country to country, the mortality of COVID-19 patients could be influenced by differences between demand, capacity and the lack of skilled ICU staff.
Similar to the Spanish reports by Iftimie et al. 26 22.9% of hospitalized confirmed COVID-19 patients died in the first wave, and this percentage was reduced to 2.7% and 3.5% in the fifth and sixth waves, respectively.The in-hospital mortality rate in five waves of COVID-19 in Mexico was 45.1% 27 .
In the current study, the in-hospital mortality rate associated with COVID-19 was lowest in the age group 0-17 years and increased in patients 65≥ years.Some studies have shown that advancing age is associated with a weakened immune system and an increased risk of comorbidities, leading to higher COVID mortality 28 .In the ongoing study, more than half of the hospitalized cases were male patients, which is consistent with other studies 18,29,30 .Age is the most important risk factor for severe COVID-19 outcomes.Patients with one or more underlying diseases are also at high risk [31][32][33] .Furthermore, the Italian study found that age and being male are separate factors that increase the risk of mortality from COVID-19, regardless of whether the patient was hospitalized or not 34 .Additionally, hospitalization during the second and third waves was linked to a lower risk of death from COVID-19 compared to the first wave.However, there was no significant difference in survival rates for patients over the age of 75 35 .In line with previous studies, the most common underlying diseases were CVD, diabetes, and hypertension 36,37 .The results of the current study suggested the association between several underlying diseases, including CVD, diabetes, BND, KD, cancer, pregnancy, and RD, and COVID-19-related mortality, which is similar to the findings of Hesni et al. 18 , Namayandeh et al. 19 , Singhal et al. 38 , Thakur et al. 39 , Gu et al. 37 , Flaherty et al. 36 and Iftimie et al. 26 .In the present study, diabetes was recognized as the second major underlying disease associated with COVID-19 mortality, which is in line with the finding of Iftimie et al. 26 .
Interestingly, the in-hospital mortality rate of COVID-19 in pregnant women in the present study was very low (1 case per 281 pregnant women).Similarly, a multicenter retrospective cohort study of COVID-19 in hospitalized pregnant women from Kermanshah province in Iran evaluated one case per 259 pregnant women 18 and a systematic review reported that the mortality of pregnant women with COVID-19 was lower than that of

Figure 1 .
Figure 1.Map of Iran showing the location of the district of Babol in the province of Mazandaran in northern Iran (Babol district is shown in Highlight, From Wikipedia, the free encyclopedia, https:// en.wikip edia.org/ wiki/ File: Iran_ locat ion_ map.svg).

Figure 2 .
Figure 2. Survival (length of hospitalization to discharge alive or mortality) of COVID-19 patients between the different waves.

Table 1 .
Characteristics and outcomes in 24,287 patients hospitalized with suspected COVID-19 by rRT-PCR status in Babol district in six waves (from March 7, 2020 to